In a current Long Term Evolution (LTE) system, a hybrid automatic repeat request (HARQ) technology is widely used. For example, in a downlink direction, an evolved NodeB (eNodeB) in the LTE system sends a data packet to user equipment (UE). When the UE correctly receives the data packet, the UE returns an acknowledgement (ACK) to the eNodeB, and when the UE does not correctly receive the data packet, the UE returns a negative acknowledgement (NACK) to the eNodeB, so that the eNodeB retransmits the data packet that is not correctly received by the UE.
In an LTE-Advanced system, a carrier aggregation (CA) technology is supported, where the technology aggregates multiple component carriers to provide larger bandwidth. Each carrier has an independent HARQ process of its own, that is, both initial transmission and retransmission that are of one HARQ process are performed on one carrier, and multiple carriers have their own HARQ processes. For example, a network device 1 and a network device 2 may send a data packet to the UE at the same time by using the CA technology, and the UE may receive the data packet from the two network devices. Based on the foregoing HARQ technology, after receiving the data packet, the UE further needs to return answer information to a network device according to whether the data packet is correctly received. However, if the answer information can be carried on only one uplink carrier due to a limitation on an uplink transmission capability of the UE, the answer information sent by the UE for the data packet that is sent by the two network devices can be received by only one network device. Therefore, for the network device 1 and the network device 2, if the network device 1 has a capability to receive the answer information sent by the UE for the data packet that is sent by the two network devices, the network device 1 transmits, to the network device 2 by means of backhaul, answer information that is in the received answer information and that is for the network device 2.
An existing CA technology is for an ideal application scenario, and ideal backhaul between network devices is required or one network device controls multiple carriers. An information exchange delay of the ideal backhaul is very short, and information may be exchanged in a timely manner. However, in actual deployment, due to an environment in which a device is located, a construction cost, and the like, the ideal backhaul between the network devices is difficult to be implemented. In a case of non-ideal backhaul, a relatively great delay may occur in information exchange between the network devices. Therefore, for a network device that is in multiple network devices implementing the CA technology and that can receive the answer information, the network device needs to transmit, to another network device, the answer information fed back by the UE for the data packet that is sent by the network devices. However, the answer information probably cannot be transmitted to the other network device in a timely manner due to a delay of the information exchange between the network devices, and data cannot be retransmitted in a timely manner. Therefore, the CA cannot be implemented.
The prior art provides a technical solution in which when only one network device schedules, for UE, physical downlink shared channel (PDSCH) transmission at a same moment, the UE feeds back, by means of quick switching of an uplink frequency channel number, ACK or NACK information for the PDSCH transmission of the network device, and two devices serve one user in a time division manner. Because there is a fixed time difference between transmission of uplink ACK/NACK and transmission of a downlink PDSCH, for example, the transmission of the ACK/NACK lags behind the PDSCH by four subframes. Therefore, the ACK/NACK is transmitted in an uplink time division manner, causing time division transmission of the downlink PDSCH. Although this manner enables the network device to acquire the ACK or the NACK in a timely manner, a peak rate that a CA technology may reach is limited, that is, a PDSCH cannot be transmitted to a same user at a same moment on two carriers, which is not real carrier aggregation.
In an existing carrier aggregation technology, one network device may control multiple carriers, but there must be a center control scheduler that can quickly obtain scheduling information of the multiple carriers. However, if one network device has multiple units that respectively control the carriers, quick information exchange among all units is required.
In the prior art, in a case in which concurrent transmission capabilities of multiple carriers for user equipment in an uplink direction are limited, for an implementation solution of carrier aggregation of two network devices or a same network device, quick information exchange needs to be performed between the two network devices or quick information exchange needs to be performed between different units of the same network device, causing a problem of hardware implementation complexity.